It has long been known to provide a single point cutting tool by the combination of a tool holder of common grade tool steel and a cutter insert of more expensive cutting material such as ceramic or tungsten carbide. Typically, such inserts are disposable items and are discarded for scrap once the cutting edges have become dull.
In order to increase the utility of such inserts, it has been the practice to form a number of cutting edges thereupon which may be successively indexed into a desired cutting position so that the effective life of the insert is significantly increased. One problem in the cutter insert industry is that of providing an acceptable number of cutting edges on a insert when a positive shear angle is desired. Conventional practice in this regard has permitted the presentation of only a four-edge positive cutting insert.
One effort to increase the number of positive cutting edges available in a disposable cutter insert is disclosed in U.S. Pat. No. 3,142,110, issued to Hertel, and entitled "Cutting Tool for Lathes and Similar Machines." There, an insert comprised of a pair of truncated pyramidal-like elements, joined at their heads, presents eight positive cutting edges along the bases of the elements. The desired relief angles are achieved by twisting the pyramidal elements relative to one another about their common central axis. While this design does offer an increased number of cutting edges, it presents a serious drawback in that the tool holder pocket for supporting the cutter insert must be specifically configured to complementally receive the complicated geometry of the insert. Moreover, the geometry of the cutter insert is such that it is unsuited for use as both a roughing and finishing tool in applications where chip control is required.
Another approach to this problem is shown in U.S. Pat. No. 3,490,117, also issued to Hertel. There, an eight-edge positive insert of the type disclosed in the U.S. Pat. No. 3,142,110 has an arcuate transition between the opposed cutting edges on each side of the insert. Semi-cylindrical blocks are required for mounting the insert within the square pocket of a tool holder.